The invention relates to an electromechanical wheel brake device for a motor vehicle.
An electromechanical wheel brake device that defines a similar system has been disclosed by EP-A-0 275 783. The prior art wheel brake device has an axially immobile threaded spindle which can be connected to a first gear by means of a switchable electromagnetic clutch. A rotatable and axially mobile spindle nut on the threaded spindle supports a second gear, which can be coupled to the spindle nut in a manner that depends on the axial force. The two gears, together with a third gear that can be driven by an electric motor 30 and engages the two other gears, constitute a differential drive. The spindle nut supports a longitudinally mobile piston for pressing a frictional brake lining against a brake body.
In a braking maneuver, the electric motor and the electromagnetic clutch are supplied with current. The rotary driven threaded spindle moves the spindle nut, which is not driven at first, in order to rapidly overcome the air play between the frictional brake lining and the brake body. With the buildup of an axial force that couples gear to the spindle nut due to the engagement of the frictional brake lining with the brake body, the spindle nut is also driven to rotate and the differential drive is activated. The buildup of brake force acting on the brake body is now produced with considerably reduced advancing speed of the spindle nut.
In order to release the wheel brake device, the electromagnetic clutch is switched off whereas the electric motor is supplied with current. The threaded spindle, which is embodied as selflocking-free and without an electromotive drive, is now driven in the opposite rotation direction by the reaction force of the wheel brake device, which results in an axial restoring of the spindle nut, which continues to be electromotively driven at first. Once the air play is adjusted, the spindle nut is also not driven and the electric motor is switched off.
The known wheel brake device is relatively costly in terms of its construction, is friction-encumbered to a considerable degree in its linkage, and requires a frequent switching of the electromagnetic clutch during normal operation. The restoring of the wheel brake device and the adjusting of the air play takes place in a largely uncontrolled manner.
In an electromechanical wheel brake device known from the Japanese abstract 06 327190 A, a threaded spindle is provided, which can both rotate and move axially, with which a frictional brake lining can be electromotively pressed against a brake body and released from the brake body. In addition, the known wheel brake device has a rotation direction independent clutch (free engine clutch) which holds the threaded spindle so that the spindle cannot rotate during the brake actuation. Only if the brake force is neutralized when the brake is electromotively released does the free engine clutch release the threaded spindle for the automatic, uncontrolled adjustment of an air play. The publication does not give any indication as to how the wheel brake device should be released in the event that there is a failure of the electric motor, nor does it disclose the manner in which the free engine clutch is actuated.
An electromechanical wheel brake device has also been disclosed by WO 96/03301. This wheel brake device is embodied as a disc brake. The wheel brake device has a brake caliper embodied as a floating caliper, which has two wheel brake linings disposed in the floating caliper on both sides of a brake body in the form of a brake disc which can be set into rotation between the brake linings. In order to press one of the two frictional brake linings against the one side of the brake disc, the known wheel brake device has a screw link actuator whose threaded nut can be driven to rotate by an electric motor and whose threaded spindle can be pressed against the one frictional brake lining. The other frictional brake lining is pressed against the other side of the brake disc in an intrinsically known manner by means of reaction forces which occur when the one frictional brake lining is applied and which are transmitted to the other frictional brake lining by the floating caliper.
In order to restore the known wheel brake device in the event of a malfunction, i.e. in the event of the failure of the control electronics or in the event of a failure of the electric motor its power supply, the known wheel brake device has a restoring spring in the form of a spiral spring which is supported with its one end against the brake caliper and which engages the threaded spindle with its other end. When the wheel brake device is actuated, this spiral spring is stretched and wheel brake device except for an acceptable residual brake force is assured in the event of a malfunction.
In order to actuate the wheel brake device, according to the invention the wheel brake device has a releasable rotation prevention device which prevents a rotation of the threaded spindle and permits a translatory movement of the threaded spindle in the axial direction through the rotating drive of the spindle nut. The wheel brake device is released by means of driving the spindle nut to rotate in the opposite direction so that an air play between the frictional brake lining and the brake body can be electromotively adjusted. A release of the rotation prevention device of the threaded spindle is only provided in event of a malfunction. The threaded spindle is connected to the detachable rotation prevention device in a rotationally secured and axially mobile manner, preferably by means of a positively engaging connection, i.e. the threaded spindle moves in the axial direction when the frictional brake lining is applied and released, whereas the rotation prevention device remains at rest, i.e. the rotation prevention device does not follow along with the shifting motion of the threaded spindle.
The wheel brake device according to the invention has the advantage that the rotation prevention device, which permits a release of the wheel brake device without the electric motor in the event of a malfunction, does not influence the efficiency and the function of the wheel brake device. Another advantage is that the electric motor can continuously adjust the air play between the frictional brake lining and the brake body to the same value independent of a wear on the frictional brake lining so that the application path and application time do not increase.
Since the rotation prevention device of the wheel brake device according to the invention does not move along with the threaded spindle, no free space has to be provided for a movement of the rotation prevention device during the actuation efficiency and the function of the wheel brake device. Another advantage is that the electric motor can continuously adjust the air play between the frictional brake lining and the brake body to the same value independent of a wear on the frictional brake lining so that the application path and application time do not increase.
Since the rotation prevention device of the wheel brake device according to the invention does not move along with the threaded spindle, no free space has to be provided for a movement of the rotation prevention device during the actuation of the wheel brake device. This saves a considerable amount of space since with a rotation prevention device that is moved along with the threaded spindle, a free space that corresponds to the thickness of the two wheel brake linings plus the air play would have to be provided for the rotation prevention device. A brake caliper of the wheel brake device according to the invention can in this manner be embodied as approx. 20 to 30 mm shorter in the axial direction, which has a considerable advantage since the wheel brake device usually has to be accommodated with extremely limited space on the inside of a rim of a vehicle wheel. Another advantage of the rotation prevention device that does not move along with the threaded spindle is a reduced friction in the actuation and releasing of the wheel brake device according to the invention, which leads to improved dynamics in the actuation and release of the wheel brake device. Particularly in the event of a malfunction, the wheel brake device according to the invention releases in an extremely short time. Another advantage of the reduced friction in the releasing of the wheel brake device according to the invention is that the residual brake force is virtually nil. Moreover, the risk of a tilting or jamming, for example due to contamination, is slight with the rotation prevention device that does not move along with the threaded spindle.
Advantageous embodiments and improvements of the wheel brake device disclosed are set forth hereinafter.